Photochemistry plays an important role in many aspects of biology, yet many mechanistic features of these reactions are poorly understood. Various aspects of ketone photochemistry are currently being studied in our laboratory. These include orientational aspects of hydrogen atom transfers; entropic control of reactivity; heterocycle formation via biradical cyclization; intramolecular energy transfer; stereochemistry of biradical reactions. To better understand these various reactions, model compounds are synthesized for photochemical reactions. Both low- and high-resolution mass spectrometry (FAB-MS, EI-MS) is used to characterize both reactants and products.